Respiratory syncytial virus and pulmonary surfactant

Revisiting the role of pulmonary surfactant in chronic inflammatory lung diseases and environmental exposure

Pulmonary surfactant is a crucial and dynamic lung structure whose primary functions are to reduce alveolar surface tension and facilitate breathing. Though disruptions in surfactant homeostasis are typically thought of in the context of respiratory distress and premature infants, many lung diseases have been noted to have significant surfactant abnormalities. Nevertheless, preclinical and clinical studies of pulmonary disease too often overlook the potential contribution of surfactant alterations - whether in quantity, quality or composition - to disease pathogenesis and symptoms. In inflammatory lung diseases, whether these changes are cause or consequence remains a subject of debate. This review will outline 1) the importance of pulmonary surfactant in the maintenance of respiratory health, 2) the diseases associated with primary surfactant dysregulation, 3) the surfactant abnormalities observed in inflammatory pulmonary diseases and, finally, 4) the available research on the interplay between surfactant homeostasis and smoking-associated lung disease. From these published studies, we posit that changes in surfactant integrity and composition contribute more considerably to chronic inflammatory pulmonary diseases and that more work is required to determine the mechanisms underlying these alterations and their potential treatability.

A nano perspective behind the COVID-19 pandemic

The global pandemic scenario has definitely pushed the scientific community to develop COVID-19 vaccines at unprecedented speed. Nevertheless, a worldwide vaccination campaign is still far from being achieved, making the usual precautionary measures as necessary as at the beginning of the outbreak. Many aspects of the SARS-CoV-2 infectious potential and disease severity do not solely rely on interactions at the molecular level but also on physical-chemical parameters that often involve nanoscale effects. Here the SARS-CoV-2 journey to infect a susceptible host is reviewed, focusing on the nanoscale aspects that play a role in the viral infectivity and disease progression. These nanoscale-driven interactions are essential to establish mitigation-related strategies.

The Bacterial and Viral Agents of BRDC: Immune Evasion and Vaccine Developments

Bovine respiratory disease complex (BRDC) is a multifactorial disease of cattle which presents as bacterial and viral pneumonia. The causative agents of BRDC work in synergy to suppress the host immune response and increase the colonisation of the lower respiratory tracts by pathogenic bacteria. Environmental stress and/or viral infection predispose cattle to secondary bacterial infections via suppression of key innate and adaptive immune mechanisms. This allows bacteria to descend the respiratory tract unchallenged. BRDC is the costliest disease among feedlot cattle, and whilst vaccines exist for individual pathogens, there is still a lack of evidence for the efficacy of these vaccines and uncertainty surrounding the optimum timing of delivery. This review outlines the immunosuppressive actions of the individual pathogens involved in BRDC and highlights the key issues in the development of vaccinations against them.

Bacillus subtilis Inhibits Viral Hemorrhagic Septicemia Virus Infection in Olive Flounder (Paralichthys olivaceus) Intestinal Epithelial Cells

Viral hemorrhagic septicemia virus (VHSV) is a highly pathogenic virus that infects a wide range of host fish species causing high economic losses in aquaculture. Epithelial cells in mucosal organs are target sites for VHSV entry into fish. To protect fish against VHSV infection, there is a need to develop antiviral compounds able to prevent establishment of infection at portals of virus entry into fish. Bacillus subtilis is a probiotic with excellent antiviral properties, of which one of its secretions, surfactin, has been shown to inhibit viral infections in mammals. Herein, we demonstrate its ability to prevent VHSV infection in olive flounder (Paralichthys olivaceus) intestinal epithelial cells (IECs) and infection in internal organs. Our findings show inhibition of VHSV infection in IECs by B. subtilis and surfactin. In addition, our findings showed inhibition of VHSV in Epithelioma Papulosum Cyprini (EPC) cells inoculated with intestinal homogenates from the fish pretreated with B. subtilis by oral exposure, while the untreated fish had cytopathic effects (CPE) caused by VHSV infection in the intestines at 48 h after the VHSV challenge. At 96 h post-challenge, samples from the untreated fish had CPE from head kidney and spleen homogenates and no CPE were observed in the intestinal homogenates, while the B. subtilis-pretreated fish had no CPE in all organs. These findings demonstrate that inhibition of VHSV infection at portals of virus entry in the intestines culminated in prevention of infection in internal organs. In summary, our results show that B. subtilis has the potential to prevent VHSV infection in fish and that its use as a probiotic in aquaculture has the potential to serve as an antiviral therapeutic agent against different viral infections.

Impact of Respiratory Syncytial Virus Infection on Host Functions: Implications for Antiviral Strategies

Respiratory syncytial virus (RSV) is one of the leading causes of viral respiratory tract infection in infants, the elderly, and the immunocompromised worldwide, causing more deaths each year than influenza. Years of research into RSV since its discovery over 60 yr ago have elucidated detailed mechanisms of the host-pathogen interface. RSV infection elicits widespread transcriptomic and proteomic changes, which both mediate the host innate and adaptive immune responses to infection, and reflect RSV's ability to circumvent the host stress responses, including stress granule formation, endoplasmic reticulum stress, oxidative stress, and programmed cell death. The combination of these events can severely impact on human lungs, resulting in airway remodeling and pathophysiology. The RSV membrane envelope glycoproteins (fusion F and attachment G), matrix (M) and nonstructural (NS) 1 and 2 proteins play key roles in modulating host cell functions to promote the infectious cycle. This review presents a comprehensive overview of how RSV impacts the host response to infection and how detailed knowledge of the mechanisms thereof can inform the development of new approaches to develop RSV vaccines and therapeutics.

Involvement of Surfactant Protein D in Ebola Virus Infection Enhancement via Glycoprotein Interaction

Since the largest 2014⁻2016 Ebola virus disease outbreak in West Africa, understanding of Ebola virus infection has improved, notably the involvement of innate immune mediators. Amongst them, collectins are important players in the antiviral innate immune defense. A screening of Ebola glycoprotein (GP)-collectins interactions revealed the specific interaction of human surfactant protein D (hSP-D), a lectin expressed in lung and liver, two compartments where Ebola was found in vivo. Further analyses have demonstrated an involvement of hSP-D in the enhancement of virus infection in several in vitro models. Similar effects were observed for porcine SP-D (pSP-D). In addition, both hSP-D and pSP-D interacted with Reston virus (RESTV) GP and enhanced pseudoviral infection in pulmonary cells. Thus, our study reveals a novel partner of Ebola GP that may participate to enhance viral spread.

Viral-Bacterial Co-infections in the Cystic Fibrosis Respiratory Tract

A majority of the morbidity and mortality associated with the genetic disease Cystic Fibrosis (CF) is due to lung disease resulting from chronic respiratory infections. The CF airways become chronically colonized with bacteria in childhood, and over time commensal lung microbes are displaced by bacterial pathogens, leading to a decrease in microbial diversity that correlates with declining patient health. Infection with the pathogen Pseudomonas aeruginosa is a major predictor of morbidity and mortality in CF, with CF individuals often becoming chronically colonized with P. aeruginosa in early adulthood and thereafter having an increased risk of hospitalization. Progression of CF respiratory disease is also influenced by infection with respiratory viruses. Children and adults with CF experience frequent respiratory viral infections with respiratory syncytial virus (RSV), rhinovirus, influenza, parainfluenza, and adenovirus, with RSV and influenza infection linked to the greatest decreases in lung function. Along with directly causing severe respiratory symptoms in CF populations, the impact of respiratory virus infections may be more far-reaching, indirectly promoting bacterial persistence and pathogenesis in the CF respiratory tract. Acquisition of P. aeruginosa in CF patients correlates with seasonal respiratory virus infections, and CF patients colonized with P. aeruginosa experience increased severe exacerbations and declines in lung function during respiratory viral co-infection. In light of such observations, efforts to better understand the impact of viral-bacterial co-infections in the CF airways have been a focus of clinical and basic research in recent years. This review summarizes what has been learned about the interactions between viruses and bacteria in the CF upper and lower respiratory tract and how co-infections impact the health of individuals with CF.

Understanding respiratory syncytial virus (RSV) vaccine development and aspects of disease pathogenesis

Respiratory syncytial virus (RSV) is the most important cause of lower respiratory tract infections causing bronchiolitis and some mortality in young children and the elderly. Despite decades of research there is no licensed RSV vaccine. Although significant advances have been made in understanding the immune factors responsible for inducing vaccine-enhanced disease in animal models, less information is available for humans. In this review, we discuss the different types of RSV vaccines and their target population, the need for establishing immune correlates for vaccine efficacy, and how the use of different animal models can help predict vaccine efficacy and clinical outcomes in humans.

The roles of direct recognition by animal lectins in antiviral immunity and viral pathogenesis

Lectins are a group of proteins with carbohydrate recognition activity. Lectins are categorized into many families based on their different cellular locations as well as their specificities for a variety of carbohydrate structures due to the features of their carbohydrate recognition domain (CRD) modules. Many studies have indicated that the direct recognition of particular oligosaccharides on viral components by lectins is important for interactions between hosts and viruses. Herein, we aim to globally review the roles of this recognition by animal lectins in antiviral immune responses and viral pathogenesis. The different classes of mammalian lectins can either recognize carbohydrates to activate host immunity for viral elimination or can exploit those carbohydrates as susceptibility factors to facilitate viral entry, replication or assembly. Additionally, some arthropod C-type lectins were recently identified as key susceptibility factors that directly interact with multiple viruses and then facilitate infection. Summarization of the pleiotropic roles of direct viral recognition by animal lectins will benefit our understanding of host-virus interactions and could provide insight into the role of lectins in antiviral drug and vaccine development.

Structure-function relationships in pulmonary surfactant membranes: from biophysics to therapy

Pulmonary surfactant is an essential lipid-protein complex to maintain an operative respiratory surface at the mammalian lungs. It reduces surface tension at the alveolar air-liquid interface to stabilise the lungs against physical forces operating along the compression-expansion breathing cycles. At the same time, surfactant integrates elements establishing a primary barrier against the entry of pathogens. Lack or deficiencies of the surfactant system are associated with respiratory pathologies, which treatment often includes supplementation with exogenous materials. The present review summarises current models on the molecular mechanisms of surfactant function, with particular emphasis in its biophysical properties to stabilise the lungs and the molecular alterations connecting impaired surfactant with diseased organs. It also provides a perspective on the current surfactant-based strategies to treat respiratory pathologies. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Toll-like receptors in neonatal sepsis

Toll-like receptors are vital transmembrane receptors that initiate the innate immune response to many micro-organisms. The discovery of these receptors has improved our understanding of host-pathogen interactions, and these receptors play an important role in the pathogenesis of multiple neonatal conditions such as sepsis and brain injury. Toll-like receptors, especially TLRs 2 and 4, are associated with necrotizing enterocolitis, periventricular leukomalacia and sepsis.

CONCLUSION

Toll-like receptor modulation may potentially be used as immunomodulators in the management of neonatal sepsis.

The Innate Immune System of the Perinatal Lung and Responses to Respiratory Syncytial Virus Infection

The response of the preterm and newborn lung to airborne pathogens, particles, and other insults is initially dependent on innate immune responses since adaptive responses may not fully mature and require weeks for sufficient responses to antigenic stimuli. Foreign material and microbial agents trigger soluble, cell surface, and cytoplasmic receptors that activate signaling cascades that invoke release of surfactant proteins, defensins, interferons, lactoferrin, oxidative products, and other innate immune substances that have antimicrobial activity, which can also influence adaptive responses. For viral infections such as respiratory syncytial virus (RSV), the pulmonary innate immune responses has an essential role in defense as there are no fully effective vaccines or therapies for RSV infections of humans and reinfections are common. Understanding the innate immune response by the preterm and newborn lung may lead to preventive strategies and more effective therapeutic regimens.

Protective effect of surfactant protein d in pulmonary vaccinia virus infection: implication of A27 viral protein

Vaccinia virus (VACV) was used as a surrogate of variola virus (VARV) (genus Orthopoxvirus), the causative agent of smallpox, to study Orthopoxvirus infection. VARV is principally transmitted between humans by aerosol droplets. Once inhaled, VARV first infects the respiratory tract where it could encounter surfactant components, such as soluble pattern recognition receptors. Surfactant protein D (SP-D), constitutively present in the lining fluids of the respiratory tract, plays important roles in innate host defense against virus infection. We investigated the role of SP-D in VACV infection and studied the A27 viral protein involvement in the interaction with SP-D. Interaction between SP-D and VACV caused viral inhibition in a lung cell model. Interaction of SP-D with VACV was mediated by the A27 viral protein. Binding required Ca2+ and interactions were blocked in the presence of excess of SP-D saccharide ligands. A27, which lacks glycosylation, directly interacted with SP-D. The interaction between SP-D and the viral particle was also observed using electron microscopy. Infection of mice lacking SP-D (SP-D-/-) resulted in increased mortality compared to SP-D+/+ mice. Altogether, our data show that SP-D participates in host defense against the vaccinia virus infection and that the interaction occurs with the viral surface protein A27.

Uteroglobulin-related protein 1 and severity of respiratory syncytial virus infection in children admitted to hospital

There are several reports suggesting that genetic factors contribute to the severity of infection with the respiratory syncytial virus (RSV). Infants hospitalized with lower respiratory tract infection (LRTI) due to RSV are at a significantly increased risk for both recurrent wheezing and childhood asthma. Uteroglobin-related protein 1 (UGRP1) is a secretory protein expressed in the airways, and speculated to have anti-inflammatory activity. The presence of the -112G/A polymorphism in the UGRP1 promoter was found to have a significant correlation with asthma phenotype. Also plasma UGRP1 levels were shown to be associated both with this polymorphism and the severity of asthma. The study population consisted of 62 previously healthy infants, ≤12 months of age, who were hospitalized with RSV LRTI, and a control group of 99 healthy adults. Genotyping was performed by restriction fragment length polymorphism. UGRP1 serum levels were determined using ELISA. There were no significant differences in the overall distribution of UGRP1 -112G/A polymorphism genotypes or alleles between the hospitalized infants and healthy adults. A comparison of serum UGRP1 concentration measured at the time of admission and discharge between patients with and without the -112A allele revealed that there was no relation between the presence of the -112A allele and serum UGRP1 in hospitalized infants with RSV infection. Furthermore, there was no relationship between severity of RSV infection and genotype or serum UGRP1 concentration. These results suggest that UGRP1 does not have a major role in the development of severe RSV infection.

Pulmonary Collectins in Diagnosis and Prevention of Lung Diseases

Pulmonary surfactant is a complex mixture of lipids and proteins, and is synthesized and secreted by alveolar type II epithelial cells and bronchiolar Clara cells. It acts to keep alveoli from collapsing during the expiratory phase of the respiratory cycle. After its secretion, lung surfactant forms a lattice structure on the alveolar surface, known as tubular myelin. Surfactant proteins (SP)-A, B, C and D make up to 10% of the total surfactant. SP-B and SPC are relatively small hydrophobic proteins, and are involved in the reduction of surface-tension at the air-liquid interface. SP-A and SP-D, on the other hand, are large oligomeric, hydrophilic proteins that belong to the collagenous Ca²⁺-dependent C-type lectin family (known as “Collectins”), and play an important role in host defense and in the recycling and transport of lung surfactant (Awasthi 2010) (Fig. 43.1). In particular, there is increasing evidence that surfactant-associated proteins A and -D (SP-A and SP-D, respectively) contribute to the host defense against inhaled microorganisms (see 10.1007/978-3-7091-1065_24 and 10.1007/978-3-7091-1065_25). Based on their ability to recognize pathogens and to regulate the host defense, SP-A and SP-D have been recently categorized as “Secretory Pathogen Recognition Receptors”. While SP-A and SP-D were first identified in the lung; the expression of these proteins has also been observed at other mucosal surfaces, such as lacrimal glands, gastrointestinal mucosa, genitourinary epithelium and periodontal surfaces. SP-A is the most prominent among four proteins in the pulmonary surfactant-system. The expression of SP-A is complexly regulated on the transcriptional and the chromosomal level. SP-A is a major player in the pulmonary cytokine-network and moreover has been described to act in the pulmonary host defense. This chapter gives an overview on the understanding of role of SP-A and SP-D in for human pulmonary disorders and points out the importance for pathology-orientated research to further elucidate the role of these molecules in adult lung diseases. As an outlook, it will become an issue of pulmonary pathology which might provide promising perspectives for applications in research, diagnosis and therapy (Awasthi 2010).

Evidence for a causal relationship between respiratory syncytial virus infection and asthma

Respiratory syncytial virus (RSV) infects all children early in life, is the most common cause of infant lower respiratory tract infections, and causes disease exacerbations in children with asthma. Episodes of lower respiratory tract infection in early life are associated with asthma development. Whether RSV infection early in life directly causes asthma or simply identifies infants who are genetically predisposed to develop subsequent wheezing is debatable. Recent studies suggest that these two explanations are not mutually exclusive, and are likely both important in asthma development. An open-label study of RSV immunoprophylaxis administered to preterm infants reduced recurrent wheezing by 50%. Clinical trials of infant RSV prevention, delay or severity reduction on the outcome of childhood asthma would confirm the causal relationship between RSV infection and asthma, and offer a primary prevention strategy.

Pattern recognition receptors and genetic risk for rsv infection: value for clinical decision-making?

Respiratory syncytial virus (RSV) causes respiratory tract infections, especially among young infants. Practically, all infants are infected during epidemics and the clinical presentation ranges from subclinical to fatal infection. Known risk factors for severe RSV infection include prematurity, age of <2 months, underlying chronic lung or heart diseases, serious neurological or metabolic disorders, immune deficiency (especially a disorder of cellular immunity), crowded living conditions, and indoor smoke pollution. Twin studies indicate that host genetic factors affect susceptibility to severe RSV infection. Pattern recognition receptors (PRRs) are the key mediators of the innate immune response to RSV. In the distal respiratory tract, RSV is recognized by the transmembrane Toll-like receptor 4 (TLR4) and adapter proteins, which lead to production of proinflammatory cytokines and subsequent activation of the adaptive immune response. Surfactant proteins A and D are able to bind both RSV and TLR4, modulating the inflammatory response. Genetic variations in TLR4, SP-A, and SP-D have been associated with the risk of severe RSV bronchiolitis, but the results have varied between studies. Both the homozygous hyporesponsive 299Gly genotype of TLR4 and the non-synonymous SP-A and SP-D polymorphism influence the presentation of RSV infection. The reported relative risks associated with these markers are not robust enough to justify clinical use. However, current evidence indicates that innate immune responses including pattern recognition receptors (PRRs) and other components in the distal airways and airspaces profoundly influence the innate immune responses, playing a key role in host resistance to RSV in young infants. This information is useful in guiding efforts to develop better means to identify the high-risk infants and to treat this potentially fatal infection effectively.

Respiratory viral infections in infants: causes, clinical symptoms, virology, and immunology

In global terms, respiratory viral infection is a major cause of morbidity and mortality. Infancy, in particular, is a time of increased disease susceptibility and severity. Early-life viral infection causes acute illness and can be associated with the development of wheezing and asthma in later life. The most commonly detected viruses are respiratory syncytial virus (RSV), rhinovirus (RV), and influenza virus. In this review we explore the complete picture from epidemiology and virology to clinical impact and immunology. Three striking aspects emerge. The first is the degree of similarity: although the infecting viruses are all different, the clinical outcome, viral evasion strategies, immune response, and long-term sequelae share many common features. The second is the interplay between the infant immune system and viral infection: the immaturity of the infant immune system alters the outcome of viral infection, but at the same time, viral infection shapes the development of the infant immune system and its future responses. Finally, both the virus and the immune response contribute to damage to the lungs and subsequent disease, and therefore, any prevention or treatment needs to address both of these factors.

Respiratory syncytial virus-induced dysregulation of expression of a mucosal beta-defensin augments colonization of the upper airway by non-typeable Haemophilus influenzae

Otitis media (OM) is a polymicrobial disease wherein upper respiratory tract viruses compromise host airway defences, which allows bacterial flora of the nasopharynx (NP) access to the middle ear. We have shown, in vitro, that respiratory syncytial virus (RSV), a viral co-pathogen of OM, reduces transcript abundance of the antimicrobial peptide (AP), chinchilla beta-defensin-1 (cBD-1). Here, we demonstrated that chinchillas inoculated with RSV expressed approximately 40% less cBD-1 mRNA and protein than did mock-challenged animals. Further, concurrent RSV infection resulted in a 10-100-fold greater recovery of non-typeable Haemophilus influenzae (NTHI) from nasopharyngeal lavage fluids, compared with chinchillas challenged with NTHI in the absence of viral co-infection. Additionally, when either: anti-cBD-1 antibody (to bind secreted AP) or recombinant cBD-1 (to increase AP concentration at the mucosal surface) were delivered to chinchillas, we demonstrated that disruption of the availability of a single AP influenced the relative load of NTHI in the upper respiratory tract. Collectively, our data suggested that effectors of innate immunity regulate normal bacterial colonization of the NP and, further, virus-induced altered expression of APs can result in an increased load of NTHI within the NP, which likely promotes development of OM.

Human genetic factors and respiratory syncytial virus disease severity

SUMMARY

To explain the wide spectrum of disease severity caused by respiratory syncytial virus (RSV) and because of the limitations of animal models to fully parallel human RSV disease, study of genetic influences on human RSV disease severity has begun. Candidate gene approaches have demonstrated associations of severe RSV in healthy infants with genetic polymorphisms that may alter the innate ability of humans to control RSV (surfactants, Toll-like receptor 4, cell surface adhesion molecules, and others) and those that may control differences in proinflammatory responses or enhanced immunopathology (specific cytokines and their receptors). These studies are reviewed. They are valuable since an understanding of the direction of a polymorphism's effect can help construct a meaningful human RSV disease pathogenesis model. However, the direction, degree, and significance of the statistical association for any given gene are equivocal among studies, and the functional significance of specific polymorphisms is often not even known. Polymorphism frequency distribution differences associated with RSV infection arising from diversity in the genetic background of the population may be confounded further by multiple-hypothesis testing and publication bias, as well as the investigator's perceived importance of a particular pathogenic disease process. Such problems highlight the limitation of the candidate gene approach and the need for an unbiased large-scale genome-wide association study to evaluate this important disease.

The immune response to respiratory syncytial virus infection: friend or foe?

The immune response to respiratory syncytial virus (RSV) infection has fascinated and frustrated investigators for decades. After adverse responses to early attempts at vaccination, it became popularly held that disease following infection was related to overly aggressive immune responses. However, recent data illustrate that severe forms of disease are related to inadequate, rather than hyperresponsive, adaptive immune reactions. Thus, recovery from primary (and perhaps later) RSV infection is dependent on the quality of innate immune responses. These findings should have enormous significance to the development of vaccines and antiviral compounds.

Respiratory syncytial virus and innate immunity: a complex interplay of exploitation and subversion

Respiratory syncytial virus causes significant disease in infants, the elderly and select groups of immunocompromised patients. Healthy individuals are also naturally infected with respiratory syncytial virus repeatedly throughout life. Therefore, safe and effective vaccines and therapies are needed. However, a number of factors have prevented development of such antiviral interventions to date. These include a failed vaccine trial, the very young age of the primary target population (neonates), the inability of natural infection to induce long-term protective immunity, and an incomplete understanding of virus-host interactions. The identification of pattern recognition receptors has led to significant increases in our understanding of induction and regulation of innate immune responses. This review will address the impact of these findings on respiratory syncytial virus research.

Pretreatment with recombinant human vascular endothelial growth factor reduces virus replication and inflammation in a perinatal lamb model of respiratory syncytial virus infection

Vascular endothelial growth factor (VEGF) is increasingly recognized as a perinatal regulator of lung maturation and surfactant protein expression. Preterm and young infants are at increased risk for pulmonary immaturity characterized by insufficient surfactant production as well as increased risk for severe manifestations of respiratory syncytial virus (RSV) infection. Innate immune components including surfactant proteins A and D, and beta-defensins have putative antimicrobial activity against pulmonary pathogens including RSV. Our hypothesis was that recombinant human VEGF (rhVEGF) pretreatment therapy would decrease RSV disease in the perinatal lamb RSV model. Newborn lambs were pretreated with rhVEGF, betamethasone, or saline and then inoculated with bovine RSV or sterile medium. Tissues were collected 5 d postinoculation, corresponding to the initiation of severe lesions and peak viral replication. In RSV-infected lambs, rhVEGF therapy increased the mean daily body temperature, decreased airway neutrophil exudate, and reduced RSV replication compared with betamethasone or saline pretreatment. Furthermore, rhVEGF therapy significantly mitigated the RSV-induced increase in surfactant protein A mRNA expression and decrease in surfactant protein D mRNA expression. In control (non-RSV-infected) lambs, pretreatment with rhVEGF increased sheep beta-defensin-1 (SBD1) mRNA expression, but no alteration in surfactant proteins A and D was detected. This novel study demonstrates that rhVEGF pretreatment mitigates RSV disease and, in addition, rhVEGF regulation of innate immune genes is dependent on RSV infection status.

Characterization of a human surfactant protein A1 (SP-A1) gene-specific antibody; SP-A1 content variation among individuals of varying age and pulmonary health

The human surfactant protein A (SP-A) locus consists of two functional genes (SP-A1, SP-A2) with gene-specific products exhibiting qualitative and quantitative differences. The aim here was twofold: 1) generate SP-A1 gene-specific antibody, and 2) use this to assess gene-specific SP-A content in the bronchoalveolar lavage fluid (BALF). An SP-A1-specific polyclonal antibody (hSP-A1_Ab(68-88)_Col) was raised in chicken, and its specificity was determined by immunoblot and ELISA using mammalian Chinese hamster ovary (CHO) cell-expressed SP-A1 and SP-A2 variants and by immunofluorescence with stably transfected CHO cell lines expressing SP-A1 or SP-A2 variants. SP-A1 content was evaluated according to age and lung status. A gradual decrease (P < 0.05) in SP-A1/SP-A ratio was observed in healthy subjects (HS) with increased age, although no significant change was observed in total SP-A content among age groups. Total SP-A and SP-A1 content differed significantly between alveolar proteinosis (AP) patients and HS, with no significant difference observed in SP-A1/SP-A ratio between AP and HS. The cystic fibrosis (CF) ratio was significantly higher compared with AP, HS, and noncystic fibrosis (NCF), even though SP-A1 and total SP-A were decreased in CF compared with most of the other groups. The ratio was higher in culture-positive vs. culture-negative samples from CF and NCF (P = 0.031). A trend of an increased ratio was observed in culture-positive CF (0.590 +/- 0.10) compared with culture-positive NCF (0.368 +/- 0.085). In summary, we developed and characterized an SP-A1 gene-specific antibody and used it to identify gene-specific SP-A content in BALFs as a function of age and lung health.

Future prospects in respiratory syncytial virus genetics

Host genes, together with viral and environmental factors, determine the susceptibility, severity and course of respiratory syncytial virus infections. The course of infection is influenced by several frequently occurring gene variants that especially appear to influence the innate immune system and the regulation of the T helper (Th) type 1/Th2 cytokine pathways. Naturally occurring polymorphisms in certain genes have been associated with a severe course of respiratory syncytial virus infection. Genetic association between interleukin (IL)-4, IL-4Rα and IL-10 polymorphisms and respiratory syncytial virus bronchiolitis differ between children younger and older than 6 months, indicating a different pathogenesis in these subsets of patients. Knowledge of host genetic variants adds to our understanding of pathogenesis, and may identify critical steps to which prevention and therapy may be directed.

Expression of select immune genes (surfactant proteins A and D, sheep beta defensin 1, and toll-like receptor 4) by respiratory epithelia is developmentally regulated in the preterm neonatal lamb

Preterm infants experience enhanced susceptibility and severity to respiratory syncytial virus (RSV) infection. Terminal airway epithelium is an important site of RSV infection and the extent of local innate immune gene expression is poorly understood. In this study, expression of surfactant proteins A and D (SP-AD), sheep beta defensin 1 (SBD1), and toll-like receptor 4 (TLR4) mRNA were determined in whole lung homogenates from lambs. SP-AD and TLR4 mRNA expression increased (p < 0.05) from late gestation to term birth. In addition, gene expression of LCM-retrieved type II pneumocytes (CD208+), adjacent epithelium (CD208-) and bronchial epithelium demonstrated that bronchiole-alveolar junction epithelium (combined CD208 +/-) had significant (p < 0.05) developmental increases in SP-AD, SBD1 and TLR4 mRNA, whereas CD208+ cells had statistically significant increases only with SP-A mRNA. Using immunofluorescence, SP-AD antigen distribution and intensity were also greater with developmental age. These studies show reduced SBD1, SP-AD, and TLR4 expression in the preterm lung and this may underlie enhanced RSV susceptibility.

Effects of RSV infection on pulmonary surfactant protein SP-A in cultured human type II cells: contrasting consequences on SP-A mRNA and protein

Respiratory syncytial virus (RSV) is the most important cause of serious lower respiratory illness in infants and children. Surfactant proteins A (SP-A) and D (SP-D) play critical roles in lung defense against RSV infections. Alterations in surfactant protein homeostasis in the lung may result from changes in production, metabolism, or uptake of the protein within the lung. We hypothesized that RSV infection of the type II cell, the primary source of surfactant protein, may alter surfactant protein gene expression. Human type II cells grown in primary culture possess lamellar bodies (a type II cell-specific organelle) and the ability to express surfactant protein mRNA. These cells were infected with RSV (by morphology and antibody binding). Surfactant protein mRNA levels determined by quantitative RT-PCR indicated a marked increase in SP-A mRNA levels (3-fold) 24 h after RSV exposure, whereas SP-D mRNA levels were unaffected. In contrast to mRNA levels, total SP-A protein levels (determined by Western blot analysis) were decreased 40% after RSV infection. The percentage of secreted SP-A was 43% of the total SP-A in the RSV-infected cells, whereas the percentage of secreted SP-A was 61% of the total SP-A in the uninfected cells. These changes in SP-A transcript levels and protein secretion in cultured human cells were recapitulated in RSV-infected mouse lung. Our findings suggest that type II cells are potentially important targets of RSV lower respiratory infection and that alterations in surfactant protein gene expression and SP-A protein homeostasis in the lung may arise via direct effects of RSV.

Acute and long-term effects of infection by the respiratory syncytial virus in children with congenital cardiac malformations

All newborn infants have limited pulmonary reserve compared with older children. This puts them at increased risk of respiratory complications, such as those associated with infection by the respiratory syncytial virus. Young children with congenital cardiac disease are particularly likely to suffer severe disease related to infection by the virus. In these children, the extreme vulnerability of the lung to pulmonary oedema is compounded by the additional burden caused by the respiratory syncytial virus. In addition to the well-documented acute pulmonary effects of infection with the respiratory syncytial virus, there may also be consequent long-term respiratory morbidity. Clinical studies have shown that infection by the virus in infancy is associated with a higher risk of developing subsequent bronchial obstructive disease. Much debate surrounds the mechanisms underlying this association. It is thought that a combined immunological and neurogenic response mechanism is likely. Prevention of severe respiratory disease in infants and young children with congenital heart disease due to infection by the virus may, therefore, offer both immediate and long-term benefits. Indeed, an increasing body of evidence supports this hypothesis, indicating a clinical rationale for prophylaxis against the virus in infancy, in order to reduce the chance of developing reactive airways disease and asthma in later life.

Viral respiratory infections in children with technology dependence and neuromuscular disorders

BACKGROUND

Viral acute respiratory infections represent a significant cause of morbidity and mortality across all ages, especially in patients with chronic underlying conditions. Although recognized anecdotally, the risks of viral infection to those children with chronic underlying conditions rendering them technology dependent, or to those children with neuromuscular disorders, have not been well studied.

METHODS

Studies of children with underlying conditions that result in technology dependence and those with neuromuscular disorders who required hospitalization for respiratory syncytial virus infection are reviewed. Additionally surveys of physician perceptions toward risk factors for severe viral illness and prevention in this population of patients are reported. Possible mechanisms to explain the increased risk of disease severity with viral respiratory infections are explored as well.

RESULTS

Current or recent use of supplemental oxygen is associated with more severe disease in children with chronic underlying conditions, especially bronchopulmonary dysplasia. Supplemental oxygen use may be a marker for several factors known to increase the severity of viral respiratory illnesses. Children with neuromuscular weakness are also likely to experience more severe disease, most likely resulting from compromised airway clearance.

CONCLUSIONS

Although the number of children who are technology-dependent or have severe neuromuscular weakness is small, their risk of severe disease after viral respiratory infection may be similar to that of premature infants or other high risk groups. A better understanding of the factors responsible for severe viral disease in these children would help create better strategies for treatment and prevention.

Innate immune responses in respiratory syncytial virus infections

Respiratory syncytial virus (RSV) is the most important viral respiratory pathogen of early life. Studies of the immune response in general (and the innate response in particular) to this agent are of interest for a number of reasons. First, severe forms of illness may be a result of enhanced immunologic responsiveness to viral constituents at the time of infection. Secondly, the immune response to RSV may consist principally of innate immune responses at the time of maximum severity of illness. Third, RSV infection in infancy may be linked via immune mechanisms to the development of childhood wheezing. Finally there are no meaningfully effective forms of therapy for RSV infection, and elucidation of the immune response may suggest new therapeutic approaches. This review will summarize our current knowledge of innate immune responses to RSV infection. Specifically we will review early interactions of the virus with surfactant proteins and Toll-like receptors, chemokine release from infected cells, cytokine release from activated inflammatory cells, activation of neuroimmune pathways, generation of dendritic cells, the release of soluble mediators of airway obstruction, and genetic polymorphisms associated with RSV-related illness.

Pathogenesis of Respiratory Syncytial Virus Infection

Respiratory syncytial virus (RSV) is recognized as the most important cause of serious lower respiratory tract illness in infants and young children worldwide causing repeat infections throughout life with serious complications occurring in the elderly and immune compromised patient. The level of disease pathogenesis associated with RSV infection is balanced between virus elimination and the nature of the immune response to infection. The innate and adaptive immune responses to RSV infection are not fully elucidated; however, significant progress has been made in understanding the virus-host relationship and mechanisms associated with disease pathogenesis. This review summarizes important aspects of these findings, and provides current perspective on processes that may contribute to RSV disease pathogenesis.

Binding and entry of respiratory syncytial virus into host cells and initiation of the innate immune response

Respiratory syncytial virus (RSV) is the most common cause of severe lower respiratory tract infection in infants and the elderly. There is currently no effective antiviral treatment for the infection, but advances in our understanding of RSV uptake, especially the role of surfactant proteins, the attachment protein G and the fusion protein F, as well as the post-binding events, have revealed potential targets for new therapies and vaccine development. RSV infection triggers an intense inflammatory response, mediated initially by the infected airway epithelial cells and antigen-presenting cells. Humoral and cell-mediated immune responses are important in controlling the extent of infection and promoting viral clearance. The initial innate immune response may play a critical role by influencing the subsequent adaptive response generated. This review summarizes our current understanding of RSV binding and uptake in mammalian cells and how these initial interactions influence the subsequent innate immune response generated.